Over the years, I've seen the very occasional mention of people fishing at Ampleforth. However, I've never seen anything concrete about it being accessible to anyone. The only firm thing I've seen was in an old Ampleforth College prospectus (online) where is said that fishing was only available to students.
Beyond that, I suspect it's a bit of an enigma, just like the nearby Gormire (which I fished once over thirty years ago).
Good luck with your quest. If you find anything positive, I'd be happy to see the details.
As it happens and I know this sounds strange but I know some of the most senior monks that run the abbey. I could if I wanted take a free holiday there but never have. The monks return to the hall where I work in a couple of weeks so I'll ask them and see what the say.... I think it's on there land as I've had conversations about fishing with them before on there lake
I fished it about 10 years ago ( if it’s the same lake, down a very quite long and very bumpy track ) I think it’s by invite or a ‘ workers ‘ perk, anyways my niece was a part-time student and wangled us a few invites ! Lake was very weedy apart from where there was a pontoon which the students swam from, stuffed with small(ish) rudd with some decent tench and from the fact that all 3 of us got bitten off playing a fish a fair amount of pike